One does not need a gang and violence to sell drugs online. Selling drugs offline, gangs and violence will get involved.
Also buying bitcoin, exchanging to Monero, installing Tor, understanding the risks, etc. Is much more work than just finding the next dealer.
Online isn't the important factor here.
> One does not need a gang and violence to sell drugs online.
Gangs and violence aren't there to support a marketplace. They don't help you find customers or customers to find you. They don't improve the efficiency of exchange. They're there to enforce outcomes. Selling drugs leads to outcomes that don't care whether the buyer and seller found themselves online.
Well that's the entire principle of price elasticity. The less costly (not only in terms of money, but also in terms of risk and time) something gets the higher the demand, at least up to a point.